1. Field of the Invention
The invention relates to a high-frequency power amplifier using amplifier elements such as field-effect transistors or bipolar transistors, and particularly to a high-frequency power amplifier for use in communication devices that process signals in a high-frequency band such as a microwave band and a millimeter wave band for mobile communication or satellite communication.
2. Description of the Related Art
In recent years, proposed are a variety of high-performance amplifier devices, such as high electron mobility transistors (HEMTs) and heterojunction bipolar transistors (HBTs), which can amplify high-frequency signals such as those in a microwave band. Such microwave amplifiers are incorporated into a variety of communication devices, such as portable telephones, in which received micro signals are amplified to have a desired power by the amplifiers.
In the conventional microwave amplifier comprising the high-performance transistors, specifically using a negative-feedback circuit connected to a source of a field-effect transistor (hereinafter, also referred to as FET), however, parasitic oscillation can be generated at a high frequency several times as high as the desired frequency for amplification. Even when normal transistors are used, such parasitic oscillation can occur. Existence of such parasitic oscillation frequency can be fatal for the amplifier, and therefore, the amplifier should be designed to be free from such parasitic oscillation.
For example, a non-patent literature 1 as listed below discloses a high-frequency power amplifier having a large number of transistors having an interdigital electrode structure for high power amplification.
Non-patent literature 1: “High-Power GaAs FET Amplifiers,” John L. B. Walker, Editor, 1993, Artech House, Inc., FIG. 2.16, Typical pattern layout of an interdigital FET and key dimensions.
FIG. 9 shows a main structure thereof, and FIG. 10 provides an equivalent circuit diagram thereof. Referring to FIGS. 9 and 10, the conventional structure includes a large number of transistors (such as FETs) connected in parallel for high power. In FIGS. 9 and 10 showing such a structure, reference numeral 10 represents a transistor per one finger, 11 a gate feed, 12 a gate RF pad, 13 a drain pad, 14 a gate finger (a gate electrode), 15 a drain electrode, 16 a source electrode, 17 a gate lead electrode, and 21 a source pad for grounding).
In such a conventional structure, a large number of transistors 10 having an interdigital electrode structure are connected in parallel, and all gate fingers 14 are directly connected in parallel to the gate feed 11 by a lead. In such a conventional structure, a large number of closed loops can be formed between the transistors connected in parallel, and such closed loops can cause parasitic oscillation to be a problem. The conventional structure as shown in the drawings has no means for suppressing the closed loop oscillation, which is generated between the transistor fingers. In case the parasitic oscillation is generated, conventionally, an external circuit such as a resister (not shown) should be added outside the amplifier in order to suppress the parasitic oscillation.
In the conventional high-power amplifier with the interdigital electrode structure as shown in FIG. 9, a large number of transistors are connected in parallel, and the gate fingers 14 are connected to the gate feed 11 to be arranged in a single row in a linear form. Therefore, a large number of closed loops can be formed between the transistor gate fingers, and such closed loops can cause parasitic oscillation closed loop oscillation). The conventional amplifier has no means for suppressing such closed loop oscillation. In consideration of occurrence of the closed loop oscillation, a external circuit (not shown) for stabilization should be added outside the transistor-forming chip in order to suppress the parasitic oscillation. Such an external circuit should form a resistance element for isolation (element isolation), which should be connected in parallel between the gate fingers.
A stabilizing circuit such as a resistance element can be formed together with the transistors in a single chip. Conventionally, however, an extra process is needed to form such a resistance element, and an extra space is needed to hold the additional resistance element or the like, so that an area of the chip can be increased.